[libcxx] Use libunwind instead of libgcc_s if libcxxabi uses it

[phosek]

Summary

When libcxxabi is configured to use LLVM unwinder, we should use it instead of always using libgcc_s.

[mgorny]

This is not that simple. Remember that libgcc_s serves both as unwinder and compiler runtime. Since libcxx is linked with -nodefaultlibs, including gcc_s is necessary to fulfill dependency on automatically generated runtime calls (__udiv* stuff, for example). I'm working on fixing that properly but it is stalled on D25338.

Also, your patch is based on outdated code. Please update your repository before submitting patches.

[phosek]

Thanks for the comment, I just rebased my change and realized that it's no longer relevant.

lib/CMakeLists.txt

if (APPLE AND LLVM_USE_SANITIZER)
   endif()
 endif()
 
 # Generate library list.
 add_library_flags_if(LIBCXX_HAS_PTHREAD_LIB pthread)
 add_library_flags_if(LIBCXX_HAS_C_LIB c)
 add_library_flags_if(LIBCXX_HAS_M_LIB m)
 add_library_flags_if(LIBCXX_HAS_RT_LIB rt)
-add_library_flags_if(LIBCXX_HAS_GCC_S_LIB gcc_s)
 add_library_flags_if(LIBCXX_HAVE_CXX_ATOMICS_WITH_LIB atomic)
 
+if (LIBCXXABI_USE_LLVM_UNWINDER)
+  add_library_flags(unwind)
+else()
+  add_library_flags_if(LIBCXX_HAS_GCC_S_LIB gcc_s)
+endif()
+
 # Setup flags.
 if (NOT WIN32)
   add_flags_if_supported(-fPIC)
 endif()
 add_link_flags_if_supported(-nodefaultlibs)
 
 if ( APPLE AND (LIBCXX_CXX_ABI_LIBNAME STREQUAL "libcxxabi" OR
                 LIBCXX_CXX_ABI_LIBNAME STREQUAL "none"))